Andre Abelian wrote:
> I am doing PCB layout that has 12v and 5v. 12v has 2A load
> and it is used to power leds and 5v is to supply logic.
> The board is only 4 layers and I am not sure about power plane
> Should I use 12v as part of power plane or 5v.

If they all have a common ground, make layer 3 the ground plane and route
everything else as needed. It's rarely necessary to use planes for power.

>
>Alan,
>
>In this case I can use 5 and 12 split mixed plane.
>
>One of my main reason I do signals in middle layers
>is to protect circuit agents reverse engineering. Most engineers give up
>when they see no traces on top or bottom of the board.
>

I think that method would only deter the most casual of
reverse-engineering efforts.

If it's really worth it to someone for them to reverse-engineer your
PCB, it would be very little trouble to depopulate a board, slap it on
a bed-of-nails tester, and extract a netlist directly from the
interconnects. In fact, for someone with sufficient time (or enough
cheap labor available), they could probably do the same thing with an
ohmmeter.

Another possibility would be to delaminate a board into its separate
layers and view the tracks directly.

Heck, it may even be possible to use Xray methods to recover the inner
layer layouts. (Given the distances involved, though, doing that may
not be feasible.)

>On Mon, 24 Nov 2008 10:44:43 -0800, "Andre Abelian" wrote:
>
> >
> >Alan,
> >
> >In this case I can use 5 and 12 split mixed plane.
> >
> >One of my main reason I do signals in middle layers
> >is to protect circuit agents reverse engineering. Most engineers give up
> >when they see no traces on top or bottom of the board.
> >
>
>I think that method would only deter the most casual of
>reverse-engineering efforts.
>
>If it's really worth it to someone for them to reverse-engineer your
>PCB, it would be very little trouble to depopulate a board, slap it on
>a bed-of-nails tester, and extract a netlist directly from the
>interconnects. In fact, for someone with sufficient time (or enough
>cheap labor available), they could probably do the same thing with an
>ohmmeter.

Indeed, if you really want to reverse engineer something reasonably
simple, their are firms in Eastern Europe who will completely reverse
engineer a PCB for you. For about $1000 US they will do a 6 layer
100x160mm PCB back to schematics unless it has any BGA packages, then
it is between $1400 and $1600.

For about $5000 they will completely recover the code in most micros too.

I have put signal layers between planes, but not for security -
mainly for noise reduction. But you still need to bear in mind the
issues of thickness balancing.

John

>Another possibility would be to delaminate a board into its separate
>layers and view the tracks directly.
>
>Heck, it may even be possible to use Xray methods to recover the inner
>layer layouts. (Given the distances involved, though, doing that may
>not be feasible.)
>
>
>Regards, Bob
>
>

> >when they see no traces on top or bottom of the board.
> >
>
>I think that method would only deter the most casual of
>reverse-engineering efforts.
>
>If it's really worth it to someone for them to reverse-engineer your
>PCB, it would be very little trouble to depopulate a board, slap it on
>a bed-of-nails tester, and extract a netlist directly from the
>interconnects. In fact, for someone with sufficient time (or enough
>cheap labor available), they could probably do the same thing with an
>ohmmeter.

Indeed, if you really want to reverse engineer something reasonably
simple, their are firms in Eastern Europe who will completely reverse
engineer a PCB for you. For about $1000 US they will do a 6 layer
100x160mm PCB back to schematics unless it has any BGA packages, then it
is between $1400 and $1600.

For about $5000 they will completely recover the code in most micros
too.

I have put signal layers between planes, but not for security - mainly
for noise reduction. But you still need to bear in mind the issues of
thickness balancing.

John

>Another possibility would be to delaminate a board into its separate
>layers and view the tracks directly.
>
>Heck, it may even be possible to use Xray methods to recover the inner
>layer layouts. (Given the distances involved, though, doing that may
>not be feasible.)
>
>
>Regards, Bob
>
>

>John,
>
>If I erase IC numbers, conformal quote the board and put the board in
>epoxy Can they still reverse engineer the board?
>
>Andre

Pretty much, yes. The first thing they do is x-ray everything and
then they know what the chips are. Then they remove the epoxy. The
formula I have seen is Methyl Ethyl Ketone (MEK), Aceton and Toluene
- most people say in equal parts. Working in a fume hood coat the
epoxy and slowly work it off. Be careful or you can dissolve the
resin in the PCB laminate.

Hard conformal coatings are usually epoxy, so off they come too.

If the x-ray technician is good then the shading in the image will
show the copper areas.

Once upon a time it was common to see people do all of those things
to protect their design. But the reality is that if someone wants
your design they will get it. One fellow I worked for many years ago
used to remove the IC part numbers and then coat the boards in a
rubber gunk that was truly disgusting to try and remove. Far worse
than conformal coating. He went to a lot of trouble, but I told him,
you aren't really protecting yourself.

He didn't believe me, but I gave a board to a friend who had access
to a low poewr inspection type x-ray. He set up and x-rayed very TTL
and linear IC he had - so now he had a set of master patterns. The he
x-rayed the board and the next day rang my boss and told him what the
IC's were. He was incredulous. Then about two days later he came back
with the complete schematic, and he had even hand built a prototype
of the circuit just to check he had gotten things like resistor values right.

The truth is there is really no easy way to protect your design. You
can keep it away from the prying eyes of the curious, but are they a
real threat to you? If your product is that important or profitable
or desirable then whatever you do will only delay the reverse
engineering experts by a few days.

Here is another example. this time about chips. I had been supplied
some supposedly Intel CPU chips recently. I was suspicious, because
the top surface didn't look right, it had a different texture to what
I would have expected. We tried two on boards - no go. So I had them
x-rayed. Within hours I knew they were fakes - they would never work
because the chip inside was something totally different. I could
actually read the manufacturers markings on the dice.

On a 4 layer there isn't too many layer stack combinations.
However (just for the persons who didn't know, not Alan) layer stacks
are standardised.
You can see this here:
www.pcbmatrix.com/Downloads/GeneralDocuments.asp
the document is named "layer configurations"

Protecting a PCB for reverse engineering is a lost of time. The real
reason why midlle layers is good to become signal layers is to avoid
interferences and EMI radiations.
There are plenty articles about this issue on the web.
The best choice is that signal layers to be defined as mixed
signal/ground planes.

Vasile

BTW Russell, for you sanity check, this email should have TECH or EE tag ?
Please put yourself the (in)appropiate tag...if you can decide which
is the "correct" one.
:)

> Personally I would do it as follows: -
>
> Layer1: signal
> Layer2: 5v plane & 12V track as required.
> Layer3: ground
> Layer4: signal
>
> The area supplied by 12V will probably be only a small portion of the PCB,
> and I would have 12V to just that area, with the rest of that layer as a 5V
> plane.
>
> However without knowing precisely what you are setting out to do, it is
> difficult to advise you properly.
>